Hiab, an operating unit of Cargotec of Helsinki, Finland, is the world´s leading provider of on-road load handling equipment. The company undergoes a rigorous protocol for testing its mobile hydraulic crane products to ensure that they meet design specifications. Testing protocols involve driving the cranes, while monitoring crane operation to verify performance. A wide range of hydraulic fluid flows are required, varying from 10 lpm to 150 lpm. If the flow is not precisely regulated, in particular at low fluid flows, the ability of the company’s cranes to make very precise lifting motions cannot be verified.

Testing system upgrade

Hiab’s old test system consisted of a main 270 kW hydraulic power unit (HPU) that feeds six test stands at 350 bar (5,000 psi) continuous pressure. Each stand can simulate either a fixed flow or a variable flow pump. Each test stand used a programmable logic controller (PLC) providing closed-loop control via a standard performance proportional valve and a pressure compensator to maintain the desired flow.

The test system worked, but Hiab’s quality control people were dissatisfied with test quality in some operating modes. In particular, the system was very sensitive to the performance of the pressure compensator device, which had difficulty regulating the target differential pressures at very small fluid flows or at high-load induced pressures. The need to perform frequent calibrations of the compensators to adjust for weakening internal springs had also become a maintenance headache. In seeking a solution, the Hiab people contacted Ihber of Zaragoza, Spain, a hydraulic company specializing in advanced customized applications, such as motion control and testing.

Hiab wanted improved test stands to "cover the complete flow range and dramatically reduce maintenance tasks," said Javier Berné, Ihber technical manager. In turn, the Ihber team proposed the installation of new proportional servo valves and high-performance, electro-hydraulic motion controllers. "Avoiding the pressure compensator means less maintenance, and use of the high performance valve and controller means better performance," continued Berné.

Two weeks during summer holidays were available to disassemble existing test stands, install the new systems (one stand is shown in Figure 1), and test and tune the controls (see Figure 2).

"Perfect performance from the first day was a must," said Berné. "We chose to use particular two-axis controllers because of their performance, the capabilities of the software that is supplied for free with the controllers, and the high level of technical support that the vendor provided was impressive."

Each motion controller (shown in Figure 3 as mounted in one test stand’s control panel) was programmed to simulate the performance of hydraulic pumps in two operating modes. Each mode employs a different sensor configuration. A programming feature called custom feedback allows the controller to easily switch between modes.

Programming advantages

The first mode simulates feeding with a fixed flow pump and a pressure-limiting valve. In this mode, the motion controller is interfaced to a servo valve, a flowmeter, and a pressure sensor. The controller is programmed using a velocity control loop with force limiting feature. In this mode, the controller runs the velocity and force proportional-integral-derivative (PID) algorithms in parallel and uses the minimum of two PID control signals to drive the actuator. In the Hiab testing application, Ihber’s engineers programmed the velocity loop to control hydraulic fluid flow to the crane, while the force loop is set to limit the applied pressure.

The second operating mode of the test system simulates a variable flow pump with load sensing and a cut-off compensator. Again, force limiting was employed in programming the controllers, with the first loop controlling differential pressure across the crane valve (using two pressure sensors, one for crane supply and another for load-sensing) and pressure limiting by the second loop.

"The use of the motion controller’s force-limiting capability allowed us to avoid transitioning between separate control modes for velocity or differential pressure and pressure regulation during system operation," said Berné. "This makes for smoother operation and minimizes test system maintenance issues."

Motion-control loop tuning

Besides supporting the programming of motion functions, the development tool software provides support for tuning motion-control loops for precise operation.

"Real-time plotting of all the motion parameters using the plot manager allowed us to very quickly understand the system and find a good and smooth response in every situation, even after an emergency stop or during the first movements, when there is entrapped air in the crane circuit," said Berné.

In addition to the motion controller, each test stand has a PLC and a touch screen human-machine interface (HMI). All elements communicate over Profibus and connect via Profibus to an additional PLC that controls the HPU and manages alarms. Using the touch screen, the user can specify the control type (fixed or variable flow), target parameters (such as flow, maximum pressure, differential pressure), and other information the controller needs. The touch screen also receives information from the controller, including actual parameter values measured during system operation.

"With the new controls in place, the performance of the system is much better than our customer expected," said Berné. "Thanks to valve and controller performance, flow control is very accurate, even with small flows, and pressure limiting works fine for light and heavy crane loads and different speeds."